This invention relates to a multilayer sliding material which is used as the material of the bearings in automobiles, ships, agricultural machines and, in particular, internal combustion engines and, more specifically, to a multilayer sliding material which excels in seizure-resisting, and, still more specifically, to a multilayer sliding material which is equipped with a thick plating layer of silver or silver alloy.
Conventionally, the material of a high-load slide bearing for internal combustion engines generally comprises a bearing layer of a copper-based alloy bonded to a steel backing layer, the inner surface of the bearing layer being electroplated with a surface layer of a lead-based-type alloy, directly or through the intermediate dam layer. The dam layer serves to prevent the tin or indium in the surface layer from diffusing through thermal diffusion into the copper-alloy layer constituting the bearing layer as the engine oil temperature rises, which diffusion often occurs if the bearing layer of a copper-based alloy is directly electroplated with a surface layer of a lead-based alloy. Since any diffusion of the tin or indium in the surface layer into the copper alloy layer extremely deteriorates the anti-corrosion and abrasion resistance properties of this lead alloy surface layer, the intermediate dam layer which serves as a diffusionpreventing layer is provided through electroplating of nickel, silver, cobalt, iron, etc., usually with a thin plating thickness of 0.5 to 3 .mu.. Regarding silver, in particular, a bearing material for aeroplanes has long been used which comprises a steel backing layer whose surface is plated with silver with an ultra-thin plating thickness of 0.1 to 0.3 mm, and a surface layer of lead or a lead alloy provided on the silver layer. In this case, however, it is necessary for the electro-plating to be effected in three or four stages, with a thickness of 0.1 to 0.3 mm so that the adhesion between the silver and the steel may be secured.
Recently, gasoline and diesel engines used in smaller automobiles, in particular passenger cars, have been used at a high-speed rotation range. Accordingly, further improvement in seizure-resisting, abrasion-resistance and fatigue-resistance properties has come to be required with respect to engine bearings. Conventionally available three-layers copper-type alloy materials provided with a lead-alloy surface layer had such drawback that premature flow abrasion of the lead-alloy surface layer was apt to occur in particular during high-speed operation, due to the oil temperature rise. In the case of those materials which employ nickel for the intermediate dam layer, the exposure of the nickel layer to the surface often gives rise to galling when the shaft comes directly into metallic contact with the nickel layer. Of course, absence of the intermediate dam layer will lead to the above-mentioned problem of surface layer corrosion or abrasion due to the diffusion of tin or indium.
As a component of the intermediate dam layer, silver is desirable in that it is softer than nickel, cobalt, or iron, and that it excels in seizure-resisting because it does not combine chemically with iron which is a main component of a shaft, so that galling occuring at the time of surface layer abrasion can be remarkably improved. Regrettably, in the case of a thickness range of 0.5 to 3 .mu., the dam effect of the Ag layer for preventing diffusion is poorer than that of nickel, etc. Moreover, it provides no substantial improvement in the seizure-resisting property during high-speed operation.